I understand that this is a difficult skill, and unfortunately many people make the mistake that if you can sing and play guitar seperately, then you can play them together. This is definately not the case. I will list a few things to do which will help your coordination of playing and singing at the same time.

1. Internalize both the vocal Melody/Rhythm and guitar accompaniment so you can play them perfectly all the way through at about any speed without stopping (seperately).

2. Once you have done this, slowly connect the two together by singing the vocal melody on "ah" while you are playing guitar so that you can get the right coordination together. You do not need to keep a distinctive tempo/rhythm. Just make sure the syllables are lined up perfectly w/ the guitar and that the pitches are correct.

3. Once you are able to do this slowly, you can begin to say the lyrics and keep a steady tempo. Make sure that everything is rhythmically and melodically correct.

If you follow these three steps then you will see a vast improvement with this difficult skill of singing and playing together.